A serious problem in document reproduction machines is a phenomena referred to as "banding". Banding is a defect observable in output copies resulting from variations in the speed of the moving photoreceptor during, for example, the exposure of the photoreceptor by a raster output scanning device such as a laser. The velocity variations create a misplacement of scan image lines in the slow scan or process direction. For many printer applications, the output copies must be virtually band free, requiring holding velocity variations to less than one percent, which is in the order of approximately one micron spatial variation at certain frequencies and for certain systems. In addition, newer printers incorporating light scanning sources, such as Raster Output Scanners (ROS) or image bars, create successive color exposure frames on a photoreceptor during a single pass. The leading edges of each of the successive color images must be in registration, within tolerances of approximately 125 microns. This precise tolerance requirement, in turn, necessitates a very accurate spacing between exposure frames. Hence, it is necessary to accurately control the velocity and position of the photoreceptor at all times during the electrophotographic process.
Heretofore, various methods have been employed to control the velocity of endless belts, such as a photoreceptor, of which the following disclosures which may be relevant:
U.S. Pat. No. A-3,581,888 Patentee: Kelly et al. Issued: June 1, 1971
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. A-3,581,888 discloses a memory system for storing positional information of pieces of ore moving, on the surface of a belt, through a scanning zone. The system includes a belt speed sensor that is preferably a magnetic pickup positioned so as to detect the passing of slugs of magnetic material fastened at spaced intervals on the belt. The output of the belt speed sensor is a pulse series which is proportional to the speed of the belt.
U.S. Pat. No. A-3,790,271 teaches a system for controlling the processing steps of an electrostatic printing machine which employs an endless photoreceptor belt. The various processes are controlled in response to control pulses. The control pulses are generated by a magnetic pickup aligned with a drive gear operatively coupled, via a drive chain or timing belt, to the photoconductive belt. Programming control of the machine processing steps are then accomplished in response to the control pulses.
U.S. Pat. No. A-3,930,725 describes a multiple sheet feeding system for an electrostatographic printing machine. Similar to U.S. Pat. No. A-3,790,271, the system employs a magnetic pickup, aligned with a drive gear used to drive the photoconductive member, to produce control signals. The control signals are used to enable sheet conveying mechanisms to properly register sheets in timed relationship with the photoconductive member.
U.S. Pat. No. A-4,045,819 teaches an audio/video recording and playback mechanism which employs a pair of recording belts, one for the video information and the other for the audio information. The video belt is described as having properly spaced holes which are detected by an electro-optical sensor to control the speed of the belt. In another embodiment, a magnetic signal of a known frequency is located on the belt, and read out by a magnetic head, in order to obtain tight speed control.
U.S. Pat. No. A-4,072,989 discloses an audio-visual presentation device which includes a magnetic cassette tape drive and a remotely controllable slide projector. A first track of the tape contains audio signals, while a second track of the tape contains synchronization signals which are used to control the remote slide projector so that the visual presentation remains in synchronization with the audio presentation.
U.S. Pat. No. A-4,263,627 teaches an electronic tachometer for determining the speed of a servo head while traversing a series of magnetic tracks on a magnetic disk. By detecting the interaction between the head and the tracks, where the head detects a maximum signal level at the center of each track, the radial speed of the head can be determined.
U.S. Pat. No. A-4,594,618 describes, as prior art, a recording mode signal (CTL) which indicates whether the recording has been done in a standard or long-time mode. More specifically, the CTL signal is read from a tape and used to reset a counter which counts pulses from a capstan motor used to drive the tape. The recording mode is determined as a function of the maximum count achieved between successive CTL signals.
U.S. Pat. No. A-4,634,404 discloses a method of controlling the rotation speed of a driven shaft by varying the diameter of one or more variable diameter pulleys about which a V-belt is entrained. A magnetic pickup, coupled to a controller, is used to sense the position of a gear affixed in rotational association with the driven pulley. The magnetic pickup generates a speed signal in response to the projections of the gear, whereby the signals represent the rotational speed of the driven pulley.
U.S. Pat. No. A-4,675,752 teaches a method and apparatus for recording or reproducing slow motion picture images. A control pulse signal (CP) is read from magnetic tape by a control head. The CP signal is then passed to a monostable multivibrator, the output of which is used to control the voltage applied to the tape drive system to reduce resonant vibration of the tape.
In accordance with the present invention, there is provided an apparatus for regulating the velocity of a belt moving in a recirculating path. The apparatus comprises a magnetic strip, located on a surface of the belt, having a plurality of magnetic signals recorded thereon, along with a magnetic detector, positioned in proximity to said magnetic strip, for sensing the magnetic signals recorded thereon, during movement of the belt, and producing an output signal in response to the magnetic signals. The apparatus also includes control means, responsive to the output signal, for regulating said driving means so that the belt is driven at a velocity which is a function of the output signal.
In accordance with another aspect of the present invention, there is provided an apparatus for monitoring the position of a member moving along a predefined path. The apparatus comprises: a magnetic strip located on a surface of the member, said magnetic strip having at least one track suitable for recording magnetic signals thereon; a magnetic read head, operatively aligned to detect the magnetic signals recorded on the track during movement of the member; and means, responsive to a frequency at which the magnetic signals are detected on the track, for controlling the speed of the member.
Pursuant to yet another aspect of the present invention, there is provided a printing machine for producing copies of an original document, comprising a movable charge retentive member including a magnetic recording strip on a surface thereof. Also included in the apparatus are processing means for generating and developing a latent image on the charge retentive member and transferring the developed image from the charge retentive member to a copy sheet, and means for controlling the operation of said processing means in response to a signal recorded on said magnetic recording strip.